1. Field of the Invention
This invention in general relates to a trouble detecting apparatus of the air-fuel ratio sensors of internal combustion engines, and more particularly to an apparatus for detecting air-fuel ratio sensor trouble by the use of an air-fuel ratio control apparatus for internal combustion engines which executes the feedback control of air-fuel ratios on the basis of air-fuel ratio signals from two air-fuel ratio sensors respectively located at a front position and a rear position of a catalyzer in an exhaust pipe.
2. Description of the Prior Art
In general, the fuel injection quantities of internal combustion engines are controlled with feedback on the basis of signals from air-fuel ratio sensors (O.sub.2 sensors and the like) located in exhaust pipes so that the air-fuel ratios of mixed air take optimum values (for example, about 14.7) according to the driving condition of the internal combustion engines.
Usually, the oxygen densities of exhaust gases decrease in the case where the air-fuel ratios of mixed gases are on a rich side when lower than 14.7, and the oxygen densities increase in the case where the air-fuel ratios of mixed gases are on a lean side being higher than 14.7, and consequently, the output signal voltage levels of the air-fuel ratio sensors vary between 0 [V] and 1 [V] according to the oxygen densities which in turn corresponds with the air-fuel ratios the typical value of which is 14.7. For example, if the air-fuel ratios are on the rich side, the voltage values of the output signals of the air-fuel ratio sensors (hereinafter referred to as "air-fuel ratio signals") increase according to the decrease of the oxygen densities.
However, in the case where a single air-fuel ratio sensor is located only on the upstream side of a catalyzer in an exhaust pipe, a high air-fuel ratio control precision is not obtained owing to the dispersion of the output characteristics (especially their operating points) of the air-fuel ratio sensors. Thus, apparatus has been proposed which is equipped with another air-fuel ratio sensor on the downstream side of the catalyzer, and which executes the feedback control on the basis of the air-fuel ratio signals on the downstream side of the catalyzer in addition to the feedback control on the basis of the air-fuel ratio signals on the upstream side of the catalyzer.
In this case, the air-fuel ratio sensor on the downstream side of the catalyzer senses the exhaust gases on the downstream side of the catalyzer which have averaged oxygen densities after catalytic reactions, and the degree of the deterioration of the air-fuel ratio sensor by the exhaust gases is reduced. Providing such a downstream side air-fuel ratio sensor makes the high precision feedback control of air-fuel ratios possible. That is to say, it can compensate for the dispersion of air-fuel ratio sensors, injectors (or fuel injecting valves) and the like, and it compensates for the changes with the passage of time of the output characteristics. Such a double air-fuel ratio sensor system is disclosed in, for example, U.S. Pat. No. 3,939,654.
FIG. 1 is a block diagram showing an example of an ordinary air-fuel ratio control apparatus for internal combustion engines which is equipped with air-fuel ratio sensors in a front position and in a rear position (or on the upstream side and on the downstream side) of a catalyzer.
In FIG. 1, reference numeral 1 designates an internal combustion engine (hereinafter referred to as "engine"); numeral 2 designates an intake pipe supplying mixed air to the engine 1; numeral 3 designates an air cleaner located at the intake aperture on the upstream side of the intake pipe 2; numeral 4 designates an intake manifold formed at the connection part of the downstream side end of the intake pipe 2 and the engine 1; and numeral 5 designates an injector, which is located on the upstream side of the intake pipe 2, for fuel injection.
Reference numeral 6 designates a semiconductor type pressure sensor sensing the pressure P in the intake manifold 4. The pressure sensor 6 measures the amount of air inhaled from the intake pipe 2 to the engine 1 through the intake manifold 4 as being proportional to the pressure P. Reference numeral 7 designates a throttle valve located on the downstream side of the injector 5 in the intake pipe 2.
Reference numeral 8 designates a throttle sensor for sensing the throttle opening degree .phi. of the throttle valve 7; numeral 9 designates an exhaust pipe leading the burned exhaust gases out from the engine 1; numeral 10 designates a catalyzer inserted into the exhaust pipe 9 and processing the exhaust gases by means of ternary processing; numeral 11 designates a first air-fuel ratio sensor located on the upstream side of the catalyzer 10; and numeral 12 designates a second air-fuel ratio sensor located on the downstream side of the catalyzer 10.
Reference numeral 13 designates an ignition coil composed of a step-up transformer, and numeral 14 designates an igniter composed of a power transistor breaking the electricity to the primary winding of the ignition coil 13. Reference numeral 15 designates an idle switch constructed with the throttle sensor 8 in a body. The idle switch 15 "turns on upon sensing the driving state of idling of the engine 1 when the throttle valve 7 is completely shut. Reference numeral 16 designates a thermistor type water temperature sensor sensing the cooling water temperature T of the engine 1; numeral 17 designates a battery as a power supply; and numeral 18 designates a key switch for starting the power-supply from the battery 17 and for ignition-driving; and numeral 19 designates a warning lamp activated activated when various malfunctions are sensed.
Reference numeral 20 designates an electrical control unit (hereinafter referred to as "ECU") controlling the drive of the injector 5 and the warning lamp 19 and the like in accordance with various driving states. The signals to be inputted to the ECU 20 as driving state designating signals are the throttle opening degrees .phi. from the throttle sensor 8, the pressure P in the intake manifold 4 from the pressure sensor 6, the cooling water temperature T from the water temperature sensor 16, idle signals D from the idle switch 15, rotation signals R based on the breaking of the electricity of the ignition coil 13, and air-fuel ratio signals V1 and V2 from each air-fuel ratio sensor 11 and 12 respectively.
The ECU 20 is operated by being fed from the battery 17 on closing the key switch 18, and the ECU 20 generates fuel injecting quantities J to the injector 5 in response to the air-fuel ratio signals V1, V2 and the drive states of the engine 1 to execute the feedback control of the air-fuel ratios, and further the ECU 20 generates signals E, which indicate the occurrences of malfunctions, to the warning lamp 19. The ignition signals for the igniter 14 may also be generated by the ECU 20.
FIG. 2 is a block diagram showing a concrete functional construction of the ECU 20. In FIG. 2, reference numeral 21 designates an input interface shaping the waveform of the rotation signal R to get interrupt signal INT; numeral 22 designates an input interface taking in the air-fuel ratio signals V1, V2, the pressure P, the water temperature T and the throttle opening degree .phi.; numeral 23 designates an input interface taking in the idle signal D; numeral 24 designates an output interface outputting the malfunction signals E, the fuel injecting signals J and the like; numeral 25 designates a power supply circuit connected to the battery 17 through the key switch 18; and numeral 30 designates a microcomputer connected with the input interfaces 21-23, the output interface 24 and the power supply circuit 25.
The microcomputer 30 comprises a central processing unit (hereinafter referred to as "CPU") calculating air-fuel ratio feedback control quantities (hereinafter simply referred to as "air-fuel ratio control quantities") in accordance with the air-fuel ratio signal V1 and V2, a freely running counter 32 for measuring the rotational periods of the engine 1 on the basis of the rotation signal R through the input interface 21 or the interrupt signal INT, a timer 33 clocking for various controls, an analog-to-digital converter (hereinafter referred to as "A/ D converter") 34 converting the analog signals through the input interface 22 (or the air-fuel ratio signals V1, V2, the pressure P, the water temperature T and the throttle opening degree .phi.) into digital signals, an input port 35 taking in the idle signal D through the input interface 23, a random access memory (hereinafter referred to as "RAM") 36 used as a working memory of the CPU 31, a read-only memory (hereinafter referred to as "ROM") 37 memorizing the operating programs of the CPU 31 and the like, an output port 38 for outputting various control signals E and J through the output interface 24, and a common bus 39 connecting each element 32-38 to the CPU 31.
The CPU 31 reads the value of the counter 32 when an interrupt signal INT is inputted through the input interface 21, and the CPU 31 calculates the rotation period of the engine 1 from the deviation between this time value and the last time value of the counter 32 to store the calculated rotation period into the RAM 36.
The output interface 24 amplifies the control signals from the output port 38 to output them as the malfunction signals E and the fuel injecting signals J.
FIG. 3 is a functional block diagram schematically showing the air-fuel ratio feedback control arithmetic operation of the prior art microcomputer 30. In FIG. 3, reference numeral 41 designates a first PI controller executing a proportional-plus-integral control (hereinafter referred to as "PI control") of the air-fuel ratio signal V1 from the first air-fuel ratio sensor 11; and numeral 42 designates a second PI controller executing a PI control of the air-fuel ratio signal V2 from the second air-fuel ratio sensor 12.
Each PI controller 41 and 42 constitutes operation means for operating each air-fuel ratio control quantity C1 and C2 on the basis of each air-fuel ratio signal V1 and V2. The second air-fuel ratio controlling quantity C2 acts as a compensating quantity to the first air-fuel ratio control quantity C1. Furthermore, the first air-fuel ratio control quantity C1 corresponds to an air-fuel ratio compensating quantity, whereby the final fuel injecting signal J to the injector 5 is controlled with the feedback of the first air-fuel ratio control quantity C1, and the second air-fuel ratio signal V2 is made to accord with a second target value VR2.
VR1 and VR2 designate respectively first and second target values for air-fuel ratio control, which are predetermined for each air-fuel ratio signal V1 and V2. Each of the target values VR1, VR2 is set to a voltage value approximately corresponding to the optimum air-fuel ratio 14.7, but the second target value VR2 may be set at a voltage value a little higher than the target value VR1 (on the rich side or in accordance with the air-fuel ratio smaller than 14.7).
FR designates,.a basic fuel quantity calculated from the pressure P corresponding to the inhaled air quantity, CF designates a fuel compensating quantity corresponding to the acceleration or the deceleration state of the engine 1 which is based on the water temperature T and the throttle opening degree .phi., KF designates an injection time compensating coefficient of the injector 5 to a target fuel quantity, and Q designates an inoperative time compensating quantity.
Reference numeral 43 designates a subtracter outputting a deviation .DELTA.V2 between the second target value VR2 and the air-fuel ratio signal V2 to input the obtained deviation .DELTA.V2 into the second PI controller 42, numeral 44 designates an adder adding the second air-fuel ratio control quantity C2 to the first target value VR1 to obtain a compensated target value VT1, and numeral 45 designates a subtracter obtaining a deviation .DELTA.V1 between the compensated target value VT1 and the air-fuel ratio signal V1 to input the deviation .DELTA.V1 into the first PI controller 41.
The adder 44 constitutes a compensating means for compensating the air-fuel ratio control quantity C1 calculated by the first PI controller 41.
Reference numeral 46 designates a multiplier multiplying the air-fuel ratio control quantity C1 from the first PI controller 41 by the basic fuel quantity FR to generate a target fuel quantity F1, numeral 47 designates a multiplier multiplying the target fuel quantity F1 by the fuel compensating quantity CF to generate a compensated fuel quantity F, numeral 48 designates a multiplier multiplying the compensated fuel quantity F by the injection time compensating coefficient KF to generate an activation time G of the injector 5, and numeral 49 designates an adder adding the inoperative time compensating quantity Q to the driving time G to generate the final fuel injecting signal J. These multipliers 46-48 and adder 49 constitute control quantity converting means for converting the air-fuel ratio control quantity C1 into the fuel injecting signal J.
Next, the concrete operation of the prior art air-fuel ratio controlling apparatus for internal combustion engines will be described with reference to FIGS. 4(1)-4(3) as well as FIGS. 1-3.
At first, as shown in FIG. 3 and FIG. 4(1), the subtracter 43 compares the second air-fuel ratio signal V2 on the downstream side of the catalyzer 10 and the second target value VR2 to generate the deviation .DELTA.V2 (=VR2-V2); and the second PI controller 42 executes the PI control of the deviation .DELTA.V2 to calculate the air-fuel ratio controlling quantity C2.
On the other hand, as shown in FIG. 3 and FIG. 4(2), the adder 44 adds the air-fuel ratio control quantity C2 or the compensating quantity to the first target value VR1 to generate the compensated target value VT1 (=VR1+C2) for the first air-fuel ratio sensor 11. Besides, the subtracter 45 compares the first air-fuel ratio signal V1 on the upstream side of the catalyzer 10 and the compensated target value VT1 to generate the deviation .DELTA.V1 (=VT1-V1); and the first PI controller 41, as shown in FIG. 4(3), executes the PI control of the deviation .DELTA.V1 to calculate the air-fuel ratio control quantity C1 for feedback.
Thus, the air-fuel ratio control quantity C1 on the basis of the first air-fuel ratio control signal V1 is compensated by the second air-fuel ratio control quantity C2 resulting in the final air-fuel ratio control quantity.
Next, the inhaled air quantity is sensed on the basis of the pressure P from the pressure sensor 6, and the basic fuel quantity FR is calculated from the inhaled air quantity. Then, the multiplier 46 multiplies the air-fuel ratio control quantity C1 by the basic fuel quantity FR to obtain the target fuel quantity F1.
Successively, the compensating quantity corresponding to the warming-up state of the engine i is calculated on the basis of the water temperature T from the water temperature sensor 16, and the acceleration or the deceleration state of the engine 1 which is based on the compensating quantity and the throttle opening degree .phi. from the throttle sensor 8 is sensed. Then, the fuel compensating quantity CF is calculated on the basis of the compensating quantity corresponding to the acceleration or the deceleration state and the like. Thus, the multiplier 47 multiplies the target fuel quantity F1 by the fuel compensating quantity CF to obtain the compensated fuel quantity F corresponding to the final fuel injection quantity.
Moreover, the multiplier 48 multiplies the compensated fuel quantity F by the injecting time compensating coefficient KF to obtain the driving time G of the injector 5, and the adder 49 adds the useless time compensating quantity Q to the driving time G to obtain the final fuel injecting signal J for the injector 5.
As described above, the air-fuel ratio feedback control is executed so that the air-fuel ratio signal V2 on the downstream side of the catalyzer 10 becomes the second target value VR2 by compensating the target value VR1 for the first air-fuel ratio sensor 11 using the air-fuel signal V2 from the second air-fuel sensor 12.
That is to say, if the air-fuel ratio signal V2 on the downstream side of the catalyzer 10 is shown to be on the lean side (where its air-fuel ratio is larger than 14.7), the fuel injecting signal J is set to be longer, and the air-fuel ratio is controlled to shift to the rich side. Also, if the air-fuel ratio signal V2 on the downstream side of the catalyzer 10 is shown to be on the rich side (where its air-fuel ratio is smaller than 14.7), the fuel injecting signal J is set to be shorter, and the air-fuel ratio is controlled to shift to the lean side. This situation will be described on the basis of the FIGS. 4(1)-(4(3) as follows. For example, if the signal V2 is on the rich side, the deviation .DELTA.V2, which is the result of the operation of subtracting the air-fuel ratio signal V2 from the second target value VR2, becomes positive. Then, the air-fuel ratio control quantity C1 is changed by introducing the compensated target value VT1, which is the result of the operation of subtracting the above-mentioned deviation .DELTA.V2 from the target value VR1 of the first air-fuel ratio sensor 11, and by making the time judged to be in the rich state be elongated. Thus, the fuel injection quantity is made to be decreased.
However, each air-fuel ratio sensor 11 and 12 has dispersion in its output characteristic in spite of being made under the control of their characteristics. Furthermore, in particular, the sensing device of the first air-fuel ratio sensor 11 on the upstream side is exposed to intense heat owing to being located nearer to the engine I than the second air-fuel ratio sensor 12, and the device of the sensor 11 is directly exposed to harmful exhaust gas ingredients which are not catalyzed by the catalyzer 10 yet. Consequently, the deterioration of the sensing device apt to happen so that changes with the passage of time of its characteristic are caused by the deterioration. On the other hand, the second air-fuel ratio sensor 12 is exposed to the exhaust gases which are lower in temperature and comparatively cleaner than those to which the first air-fuel ratio sensor 11 is exposed, and the changes with the passage of time scarcely happen. Hereinafter, the problems due to the dispersion of the output characteristics of the first air-fuel ratio sensor 11 and the deterioration of the devices will be concretely described.
FIGS. 5(2) to 5(4) are wave form charts showing the output response characteristics of ordinary air-fuel ratio sensors in the case where their air-fuel ratios (or A/F), a typical wave form of which is shown in FIG. 5(1), are made to be changed compulsively. Reference mark Va, shown in FIG. 5(2), designates an air-fuel ratio signal of an air-fuel ratio sensor the characteristic of which is central among the other characteristics; and air-ratio signals of air-ratio sensors, the characteristic of one of which is dispersed from the central characteristic and the sensing device of the other of which deteriorates, are designated respectively by the reference marks Vb and Vc, which are shown in FIGS. 5(3) and 5(4) respectively.
as shown in FIGS. 5(1)-5(4), when the air-fuel ratio changes to the lean side or to the rich side, which is divided by the border of the target value 14.7, the air-fuel ratio signal Va of the air-fuel ratio sensor having the central characteristic responds behind about 100 milli-seconds; the air-fuel ratio signal Vb of the air-fuel ratio sensor, the response of which from the rich side to the lean side is late, responds behind about 200 milli-seconds; and the air-fuel ratio signal Vc of the air-fuel ratio sensor, both the responses of which from the rich side to the lean side and from the lean side to the rich side are late, responds behind the maximum 1.0 second.
FIGS. 6(1)-6(4) are wave form charts showing the air-fuel ratio feedback control operation in the case where the air-fuel ratio sensors having each output characteristic of the air-fuel ratio signals Va-Vc shown in FIGS. 5(2)-5(4) respectively are used as the first air-fuel ratio sensor 11.
Herein, the first target value VR1 for the first air-fuel ratio signal V1 is designated by .alpha., and the compensated target value VT1 is designated by .beta., then the case where the first target value VR1 is compensated from .alpha. to .beta. with the air-fuel ratio control quantity C2 based on the second air-fuel ratio signal V2 will be described. That is to say, the case will be described where the second air-fuel ratio signal V2 is on the richer side (or its voltage value is higher) than the second target value VR2, then where the air-fuel ratio is controlled by the feedback to the lean side by decreasing the compensated target value VT1 by adding the negative air-fuel ratio control quantity C2 to the first target value VR1.
In FIGS. 6(2)-6(4), reference marks T.alpha. and T.beta. designate times during which the air-fuel ratio signal V1 is judged to be on the rich side by comparing the air-fuel ratio signal V1 with the target values .alpha. and .beta. respectively; mark T.alpha. designates the rich-judged time in the case where the target value is set to be .alpha.; and mark T.beta. designates the rich-judged time in the case where the target value is set to be .beta..
For example, the rich-judged time of the air-fuel ratio control quantity C1 based on the air-fuel ratio sensor 11 having the air-fuel ratio signal Va (or the central characteristic), as shown in FIG. 6(2), is compensated to T.beta. longer than T.alpha., then the target value of the air-fuel ratio is changed to the decreasing side (or the lean side of the air-fuel ratio), as shown in FIG. 6(2), and consequently, the air-fuel ratio can be changed to be lean.
In the same manner, the rich-judged time using the air-fuel ratio sensor which outputs the air-fuel ratio signal Vb or Vc, as shown in FIG. 6(3) or 6(4) respectively, changes to T.beta. longer than T.alpha. by compensating the target value of the first air-fuel ratio signal V1 from .alpha. to .beta..
at this time, since the response times are different as shown in FIGS. 5(2)-5(4) owing to the dispersion of the output characteristics of the air-fuel ratio sensors 11 such as each air-fuel ratio signal Va-Vc, the change quantities (T.beta.-T.alpha.) of the rich-judged times differ according to the cases of each air-fuel ratio signal Va, the central characteristic, Vb and Vc, as shown in FIGS. 6(2)-6(4).
As described above, in the case where the output characteristics of the first air-fuel sensors differ, the change quantities of the rich-judged times T.alpha. and T.beta. differ in obedience to the change quantity (.beta.-.alpha.) of the first target value VR1 in accordance with the second air-fuel ratio control quantity C2. Consequently, the change quantities of the air-fuel ratio control quantity C1 differ in obedience to the air-fuel ratio control quantity C2, which is the compensating quantity of the first target value VR1, and differ in accordance with the differences among the output characteristics and response times of the first air-fuel ratio sensors 11. That is to say, in the case where the air-fuel ratio sensor 11 which outputs the air-fuel ratio signal Va (being central characteristic) is used, the response time is comparatively fast, and consequently, the change quantity of the rich-judged time to the air-fuel ratio control quantity C2, being the compensating quantity of the first target value VR1, becomes small. As the result, appropriate air-fuel ratio control can be executed by changing the air-fuel ratio control quantity C2 as the compensating quantity of the first target value VR1.
Furthermore, in the case of the air-fuel ratio signal Vb, the response in changing from the lean side to the rich side is late, but the response in changing from the rich side to the lean side is comparatively fast, and consequently, appropriate air-fuel ratio control can be executed by compensating the first target value VR1 with the air-fuel ratio control quantity C2 through the air-fuel sensor 12.
Furthermore, in the case where the air-fuel sensor 11 outputting the air-fuel (signal Vc is used; since both the responses from the rich side to the lean side and from the lean side to the rich side are later than those of the air-fuel sensor, outputting the air-fuel signal Vb, the change quantity of the rich-judged time becomes larger, and T.beta. becomes longer even if the air-fuel ratio control quantity C2 is changed, and consequently, the compensating quantity of the target value VR1 with the air-fuel ratio control quantity C2 becomes unacceptable for the 1.0 compensating quantity of the target value VR1 actually required, then the appropriate air-fuel ratio control cannot be executed. In particular, since the first air-fuel ratio sensor 11 on the upstream side of the catalyzer 10 is apt to deteriorate, the sensor 11 causes errors such that the changes of the : air-fuel ratio control quantity C1, and thus appropriate air-fuel ratio control, can not be attained.
Because the prior art air-fuel ratio control apparatus for internal combustion engines is constructed as mentioned above, it has a problem that it becomes difficult to execute appropriate air-fuel ratio control especially in the case where the air-fuel ratio signal is Vc owing to the dispersion of the first air-fuel ratio sensor and the changes of the characteristic due to the changes with the passage of time. Besides, in the case where the air-fuel ratio signal is Vb, appropriate air-fuel ratio control can be ensured, but it is better to detect the condition as being in trouble since the first air-fuel ratio sensor 11 itself is abnormal. On the other hand, it is desirable to judge the first air-fuel ratio sensor to be-out of order in the cases such that the magnitude of the air-fuel ratio signal V1, which is the output signal of the first air-fuel ratio sensor 11, changes, and that the change point of the first air-fuel ratio signal V1 shifts, whereas there is no description about such cases above.